This invention relates generally to AC magnetic tracking systems and, in particular, to the use of a modulate signal in conjunction with distortion compensation.
Position and orientation tracking systems (trackers) are well known in the art. They include, for example, AC electromagnetic, inertial, comboxe2x80x94consisting or two different trackers, e.g., optical and magnetic, and others. AC electromagnetic trackers have definite advantages over other kinds: they give highest solution/update rate with high accuracy, not affected by obstructed field of view (versus optical), do not require reference sensor/unit and drift stable (versus inertial), not affected by Earth""s magnetic field and ferrous materials (versus DC magnetic). The main disadvantage of AC trackers is their susceptibility to the electromagnetic distortion due to the field of eddy currents induced in the conducting materials in the vicinity of a motion box.
AC Magnetic Tracking systems are very susceptible to distortion due to eddy currents in conductive materials in or near the motion box. To overcome this phenomenon Magnetic Trackers require costly and time-consuming calibration/mapping to be able to function correctly in the distorted environment. The electromagnetic coupling which creates these eddy currents is strongly dependent on the frequency of the transmitted AC magnetic field. In addition, eddy currents are phase shifted with respect to the Magnetic Tracker source drive current that generates the magnetic field.
One of the options to compensate for electromagnetic distortion in AC trackers is mapping, i.e., measurement of the magnetic field profile in the multiple points of the volume of interest (motion box) prior to the actual tracking, see co-owned U.S. Pat. No. 6,377,041 and references therein. While mapping may be done fast and accurately any changes in the motion box will require repeating of the mapping procedure.
Another approach allows the AC tracker to trace moving metal (distortion) by measuring the signal without distortion (acquiring baseline signals) and then comparing this signal with one in the presence of distorting object(s) by measuring the phase error of the received signal. While such a system may work it is not always practical to acquire baseline signal without distortionxe2x80x94in many cases, for example an aircraft cockpit, the distortion is always present.
Yet another approach introduces at least two frequencies per source channel and uses the difference in responses to compensate for the eddy current distortion. This approach requires a guess about the eddy currents loop geometry, and efficiency of the distortion compensation and operation frequencies depends on a kind or distorted environment (guess about physical characteristics of distorting materials) where the system will be working. In addition this approach requires a comparatively wide band receiver (sensor and ADC processing) thus reducing noise stability.
Methods and apparatus for distortion compensated AC tracking described in co-owned patent application Ser. No. 09/430,978 and U.S. Pat. No. 6,369,564, and for wired and wireless sensors use witness sensors to get real time information about distortion, and uses in addition a signal from the resonantly tuned wireless passive sensor phase shifted with respect to the source and distorters.
The system and method described herein generates a modulated signal (e.g. single tone FM or AM) and performs on the tracker DSP spectral and phase analysis of the signal received by the sensor. The algorithm allows distinguishing between the direct sourcexe2x80x94sensor response and the response due to eddy currents, thus allowing elimination of effects of the electromagnetic distortion. This, in turn, eliminates the need for a calibration/mapping, which has proven to be the main obstacle for the wide application of AC electromagnetic tracking systems.
The new method and system disclosed below do not require witness sensor (but may be used in a combination with them), works in a narrow frequency band that ensures noise stability, may have high operation frequencies (e.g. about 50 kHz) that gives high quality of the signal and increased operation range, as well as high solution update rate, performs real time (each frame) distortion compensation without any prior knowledge about physical properties of distorters. At the same time the new system preserves all known advantages of the AC trackers.
The approach finds applicability in head tracking systems and helmet-mounted displays for fighter aircraft, head trackers for armored vehicles, medicalxe2x80x94guided surgery and biopsy. The same technology is applicable for remote sensing.